A separable threaded fastener which is tightened to hold two objects securely together with an axial preload on the fastener. The structure of the fastener inherently minimizes mechanical shock loads on contiguous structure which are developed when elements of the fastener are abruptly separated from one another.
Separable fasteners are widely used in many fields, especially in the aerospace field where objects are held together until a specific time when they must be reliably separated so one of the objects may be discarded, released, or deployed. In the early days of the aerospace field, reliability of separation was the primary objective. If the fastener failed to release on command, an entire project would be imperilled. For this reason redundant fasteners were often used as a back up.
Reliability is still the primary concern, but after it was solved, as it has been with improvements in actuators, attention was turned to reduction of the mechanical shock which is unavoidably exerted on surrounding structure when the fastener is released. Although fairly large shock loads can be tolerated in heavier structures, the development of very lightweight structures such as spacecraft antennas has lowered the tolerance for such forces. Many efforts have been made to provide fastener constructions which can reduce this load.
Energy which is potentially a source of mechanical shock is necessarily stored in the fastener system when it is coupled to objects that it is to hold together. Customarily one part of the fastener, usually its threaded nut, is fixed to one of the objects. A headed, threaded bolt engages the other object. It is threaded into the nut and tightened. The resulting axial tensile preload stored in the nut/bolt combination is essential to the tightness of the joinder, but also must be expended when the fastener elements are separated. The resulting shock is an abrupt axial force.
Yet another source of shock comes from the segmented threaded part of the nut. Customarily the threads are formed on inside surfaces of segments that are held against the bolt by an external removable retainer. When the fastener is to be released, a retainer moves away from the segments and they move, away from the bolt to release it. But they move rapidly and stop abruptly, thereby contributing to the shock load.
Still another source of shock is the actuator itself. The most convenient, and most frequently used actuator, is an explosive, gas-generating charge which can be detonated by an electric current. Such actuators are well-known, and are very reliable. They must be, if they are to be used in man-safe applications.
It is evident that a shock load will be generated by their detonation. Attempts have been made to eliminate or reduce this shock. One example is the use of shape-changing elements which rely on change of temperature for their actuation. Nitinol alloys and some waxes are known examples. These will indeed function for their intended purpose, but they require substantial electrical current and time. This is allowable on the ground and in some flight applications, but electrical current is in short supply in deployed spacecraft, and quick separation at a closely defined time is usually required. Extended time for actuation is often not available. Such arrangements are unsuitable for systems which must use little current and must function quickly. The objective therefore must be to reduce the size of the charge, and thereby reduce the shock load. The fastener of this invention requires a significantly-reduced force for separation.
As the aerospace industry matures and production changes from producing prototype and experimental parts for first generation products to production runs for craft expected to be built in the many hundreds, cost becomes ever more important. Surprisingly there are many uses for separate fasteners which may be used over again, perhaps 20 times in the course of assembly and testing. Prior art fasteners are poorly adapted for such re-use, or for that matter, to the less-expensive constructions attainable with long production runs. The opportunity to use the same fastener more than once is a strong financial objective.
It is an object of this invention to provide a fastener which constitutes an improvement in all of the characteristics described above.
A fastener according to this invention includes an externally threaded bolt and an internally threaded nut. The bolt is restrained to one object to be joined. The nut is restrained to the other object to be joined by a housing which itself is attached to the other object. The nut comprises a plurality of separate segments assembled around the bolt to form a thread. The segments are held in an assembled configuration by a retainer ring which is reciprocally mounted in the housing. The above are features of many prior art separable fasteners.
According to this invention a relief element is placed between the segments and the structure associated with the xe2x80x9cotherxe2x80x9d object. In this structure, it is the housing. Drawing down the nut segments will press them against the relief element and enable the nut to resist rotation and permit axial tensile preload to be established in the bolt. According to this invention, the relief element includes a stator and a rotor. The stator is restrained in the housing against rotation. The rotor is rotatable. The stator and rotor are co-axial.
A ramp surface is formed on the stator and on the rotor. They are complementary, and extend arcuately around the axis. Each has a ramp angle such that they form surfaces which substantially abut one another, at a ramp angle.
The ramp angle is steeper than a locking angle, so that an axial compressive force will exert a rotational force on the rotor, the stator being keyed to the housing to prevent its rotation. Rotation of the rotor will result in a lessening of the total axial thickness of the relief element, and will thereby relieve the axial preload.
According to a preferred but optional feature of the invention, the stator and the rotor are keyed together by reciprocable pins that prevent rotation of the rotor prior to separation of the fastener. The pins permit relative rotation when withdrawn to allow the rotor to turn and reduce the axial thickness of the relief element. Then the segments can be cammed away from the bolt and the fastener will be released.
Accordingly much of the shock force and of the relieved axial preload is converted from an axial direction into rotation, which does, in fact reduce the shock on the surrounding structure by a considerable amount.
Also, according to this invention, force required from the actuator is reduced to that of pulling the pins, a very small force indeed.
The above and other features of this invention will be fully understood from the following detailed description and the accompanying drawings, in which: